1. Field of the Invention
The present invention relates generally to broadband wireless access systems, and more particularly to a redundancy scheme for a radio frequency front end in a broadband wireless access system.
2. Description of the Related Art
Point to multi-point fixed broadband wireless access systems over MMDS networks are currently known in broadcast situations. These networks operate over licensed bands including the MMDS band (2,150 to 2,162 MHz), the WCS band (2,305 to 2,360 MHz) and the ITFS/MMDS bands (2,500 to 2,686 MHz).
One known cable based broadband access system, which operates at a range of between 50 MHz and 864 MHz, but not in the MMDS, WCS, or ITFS/MMDS bands, is the data over cable specification system, which is specified in the Data Over Cable System Interface Specifications (DOCSIS), the disclosures of which are herein incorporated by reference. An overview of a DOCSIS system is depicted in FIG. 1. A Cable Modem Termination System (CMTS) 10 communicates with a wide area network 20, such as the Internet. The CMTS 10 can transmit signals from the wide area network 20 along a cable network 30 through cable modems 40 to a subscriber's LAN or computer 50. The subscriber's LAN or computer 50 data messages can be transmitted to the wide area network 20 through the cable modem 40 along the cable network 30 to the CMTS 10.
In point to multi-point broadband wireless access systems, one central end-point, e.g. the head-end, communicates through a bi-directional link or links with multiple end-points, e.g. the nodes. The number of nodes in communication varies in time and can be none, one or two or more at any specific time.
The link(s) between the head-end and the nodes are combined into one or more channels. The signal path from the central end-point to the nodes is referred to as downstream, while the signal path from the nodes to the central end-point is referred to as upstream.
A single upstream channel can be used to deliver information from a node to the head-end or from the head-end to a node or a group of nodes. If a single upstream channel is used for communication from the nodes(s) to the central point, then only one end-point can send information on the single upstream channel at any one time.
Wireless broadband access systems are also known in the art. One problem with many wireless receiver systems used in wireless broadband access systems is device failure in the components of the RF front end. This is an extremely significant problem for the wireless hub, where there are a number of receivers for each sector. Failure of an individual front end unit could render a whole sector useless.
Referring to FIG. 2, the downstream redundancy is based at the Wireless Modem Termination Station (WMTS), where two (2) or three (3) modulators 201(a)-(c) each connect to the same number of transmitters (up converters) 202(a)-(c), where each transmits in a separate downstream channel. This system may utilize a tuned combiner 203, to combine the signals from the transmitters 202(a)-(c). A power amplifier 204 drives the combined signal over the air. This configuration can also include an agile up converter as a backup (not shown). This system enables doubling of the downstream data rate for normal operation. In the case of failure (at a modulator or at a transmitter), the modem will stop receiving the particular frequency, and automatically will switch to a second frequency. The advantages of this approach include simplicity, doubling the downstream capacity in normal operation, the use of multiple downstream modulation schemes and/or bit rates, and optimization of the maximum capacity according to the receiving capabilities of the modem (distance, interference, multipath, etc.). Furthermore, it is advantageous to control the transmitter (frequency and power level) by the WMTS. For instance, an up converter transmitter manufactured by ADC, model 5100 (and others), has SNMP control.
In the system of FIG. 3, an upstream redundant front end scheme is shown. Failure in the upstream can be determined at the WMTS 304 end based on the failure to receive an answer from all the modems on a specific upstream frequency. The above concept enables self-redundancy and self-healing. Alternative failure decision criterion includes combining the RF test point signals from the transmitters, and connecting them to a special modem used for downstream “receive only” testing. This upstream criterion can be used together with the downstream criterion to distinguish between failure in the RF equipment or in the antenna(s) at the basestation.
Upstream coverage uses a sectorized antenna for reception (up to 24 sectors). Each receiving sector includes external equipment (antenna, band pass filter, low noise amplifier (LNA), down converter) and a receiver. Each sector can use multiple upstream frequencies, by splitting the received signal from the down converter, and delivering it to separate receivers at the WMTS (FIGS. 3 and 4 describe a basic configuration having four frequencies per sector). The WMTS includes eight upstream receiver cards, each of them including four receivers.
Referring now to FIG. 4, a front end receiver including full redundancy and antenna diversity is depicted. This configuration is based on the concept of delivering full redundancy without any point of failure. The schematic shown depicts a full redundancy configuration for four sectors. The configuration includes doubling the whole chain, from the antenna to the WMTS receiver, for each sector. The advantages of the scheme depicted in FIG. 4 include:                1. It enables implementation of antenna diversity (dual antennas per sector);        2. Full upstream redundancy, allowing the option of doubling upstream capacity (for high penetration);        3. No single point of failure, and ease of redundancy control (development of switching mechanism is not required).        
However, the scheme of FIG. 4 includes several disadvantages including doubling the antenna weight and wind load required for construction of the tower (e.g. increasing the number of required antennas for 16 sectors) and most importantly the high cost due to the requirement of 32 down converters and antennas for 16 sectors.
It should be noted that in FIGS. 2, 3 and 4, the receivers are part of the WMTS, while the down converters are not.